A conventional projection system is usually as follows: imaging light emitted from a projector is projected on a projection screen, and viewers observe the light reflected from the projection screen as an image. Specific examples of projection screens for use in such conventional projection systems include white-colored paper or cloth materials, and plastic films coated with ink that scatters white light. Besides, higher-quality projection screens that comprise scattering layers containing beads, pearlescent pigments, or the like and control the scattering of imaging light by the scattering layers are now commercially available.
The above-described projection screens are usually so made that the scenes behind the projection screens cannot be seen through them to provide higher-contrast images. There are, however, some uses that demand transparent projection screens through which the scenes behind them can be seen. Specifically, for example, in the case where a projection screen is attached to a show window or the like, calculating upon an eye-catching effect, it is necessary that the scene behind the show window can be seen through the projection screen at the time when no image is displayed on the projection screen. There is, therefore, a demand for a transparent projection screen that can display an image with high visibility and through which the scene behind it can be seen.
Since such a transparent projection screen is required to be highly transparent at the time when no image is displayed and also to achieve excellent image display even under bright environmental light, a screen using a hologram (holographic screen) is widely used as the transparent projection screen. Holographic screens are suitable for use in shops with show windows as described above, as well as places of entertainment or exhibition, and so forth.
In addition to commonly used transmission-type screens (screens in which imaging light, as transmitted light, is observed from rear side), reflection-type screens (screens in which imaging light, as reflected light, is observed from front side) have been proposed for the above-described holographic screens (Japanese Patent Laid-Open Publication No. 222512/1997). However, these holographic screens, whether they are of transmission type or of reflection type, are supposed to be viewed only from one side. That is to say, viewing imaging light from both sides of a screen is, in general, beyond our conception.
Japanese Patent Publication No. 3482963 discloses a transmission-type holographic screen from whose both sides imaging light can be viewed when combined with a half mirror. However, although the screen described in this patent document can thus display images on its both sides, the viewable images are merely identical ones, and it is theoretically impossible to display on the screen two different images, one on each side of the screen.
Even in the above-described transmission- or reflection-type holographic screens, it is possible to display on the screen two images, one on each side of the screen, if two imaging light beams are simultaneously projected on the screen from the front and the rear at different angles. In this case, when the two imaging light beams that are projected on the screen from the front and the rear are made to carry different images, it is possible to display on the screen two different images, one on each side of the screen. However, in this case, it is necessary to place two projectors on the respective sides of the screen. Moreover, it is necessary to conduct positioning of the projectors and the screen, and also to make the timing of displaying an image on one side of the screen coincide with the timing of displaying another image on the other side of the screen. The simultaneous projection of two imaging light beams is thus disadvantageous in that the practical installing operation is considerably complicated.
Besides, there exists the following problem with holographic screens themselves: since holograms have wavelength selectivity but no polarized-light selectivity, they cannot separate imaging light from environmental light, which makes it difficult to sharply display images on holographic screens under bright environmental light.